In biomedical engineering, bio-impedance is the response of a living organism to externally applied electric current. Bio-impedance or body impedance is a measure of the opposition to the flow of that electric current through the tissues, the opposite of the electrical conductivity. The measurement of the body impedance of humans and animals has proved useful as a non-invasive method for measuring blood flow, body fat, BMI (body mass index) and body composition.
One method of measuring the body impedance uses electrodes. A fixed excitation current (either AC or DC) is injected in a human body through a pair of excitation electrodes. A pair of sense electrodes is coupled to the human body. A sense circuit measures a voltage difference across the sense electrodes. The voltage difference corresponds to the impedance of the human body.
An impedance associated with each electrode of the pair of excitation electrodes and the pair of sense electrodes effects the accuracy of the measured body impedance. Traditional devices does not take into consideration the high range of impedance associated with the electrodes, because these devices provide large electrodes which allow a large area of contact between the electrode and the human body. However, modern consumer devices like cell phones need to take into consideration the impedance associated with the electrodes or mismatched impedances between the electrodes.
In a calibration method, the body impedance is calibrated with respect to known resistors that are measured using the sense circuit. However, the calibration method suffers from inaccuracies owing to the impedance associated with the electrodes. The impedance associated with the electrodes is present when the body impedance is measured. However, the impedance associated with the electrodes is not taken into consideration when the known resistors are measured. Thus, this leads to inaccuracies in the measurement of the body impedance.